Bonsai, what is a Bonsai.
Bonsai is the art of miniaturisation, it is a name given to any plant that is growing in a container and is maintained in a smaller size than its natural growth method, the Japanese have refined the techniques of Bonsai to such an extent that they are now able to create entire landscapes in miniature with incredible detail.  The ultimate bonsai is one that when viewed it will resemble a huge tree grown naturally in the wild, with gnarled trunk, branches and tiny leaves.  To the the Chinese a bonsai can be a contorted misshaped tree representing a shape or symbol rather than a natural tree.  To the lazy gardener a bonsai may often unintentionally be that plant in the pot that should have been planted into the garden 10 years ago and has had to survive on total neglect. 

What plants can be turned into a Bonsai
Any normal plant can be turned into a bonsai if it can exists in a smaller than normal size and will live in containers many years, even grasses can be kept as a bonsai.  The idea behind bonsai is to create an attractive miniature plant in a pot.  The plant can be shaped and styled to suit individual tastes and is most often made to appear as a miniature tree, to be able to achieve this the plant must be able to be pruned regularly and so easily managed  shrubs or trees are generally chosen for bonsai.  Avid Bonsai growers will often experiment to the limit with difficult plants.   The essential rule in bonsai is to understand the plant first then the rules of Bonsai (if they really exist) .  The questions that must be asked are, can the roots be trimmed, can the branches be cut, will the plant survive in a shallow pot, is it indoor or outdoor.

Where can a Bonsai be kept
This depends entirely on the plant chosen and how much care will be given bu the grower.  The most popular bonsai are small well shaped trees that are kept in shallow pots.  Shallow pots dry out easily and if you have a large plant in a small pot you can expect to have to water it up to three times a day if it is kept out in the full sun in summer, in the shade at least once a day. 
Most bonsai growers install automatic watering systems or have some way of ensuring the plant cannot dry out. 
A bonsai should be treated in the same way as the plant species would naturally.
An outdoor plant should be kept outdoors.
A shade loving plant should be kept in the shade.
A pine tree should be in almost full sunlight except for the hottest part of the year. 

Can bonsai be kept indoors
The best answer to this is if you can find a plant that will naturally grow in caves then by all means it will make a great indoor plant and possibly a bonsai, the harsh reality is that plants were not designed for caves and even the best indoor plants available today only just survive.  The recipe for success as an indoor plant is good sunlight (direct) and fresh air movement, additional humidity will also help.  Without these factors all but the hardiest of indoor plants are doomed to a bare survival state.  Of all the indoor plants available only the fig trees (Ficus) have stood out as good indoor bonsai that will survive in most indoor conditions and thrive if given good light and air movement. 

Other outdoor tree types used as bonsai can only survive indoors for a few weeks and must be moved back outside to recover.  Some trees can survive in sunrooms if the light is good and air movement adequate.  All members of the conifer family should not be brought indoors into heated areas over the winter period while they rest. 

How is a Bonsai kept small
A Bonsai is a living growing plant that will continue to grow until the day it dies, however the purpose of creating a bonsai is to create a miniature effect and so maintain the plant at its present size for the rest of its natural life.  When a plant is chosen a decision must be made as to how big that plant will be allowed to grow.  Once this size has been reached the next 200 to 300 years of your time will be spent improving the shape of the plant within that size.  Many people create Bonsai without realising it.  A hedge or a topiary cut every year is the same as maintaining a bonsai.  If a stem grows 10cm then 9 cms is cut off, after 10 to 30 years this stem may be out of proportion to the tree size so the branch may be cut off or shortened.  If the plant is in a 10cm pot the idea is to keep it at this size for as long as possible, some plants can be kept over 100 years this small.  If it is in a 60cm pot then its proportions will match that pot and at this size most common bonsai species can be kept for many hundred years.  A common term used in Bonsai is sacrifice branches, these are the first branches on your bonsai or particular branches used to allow the stems to thicken and are then removed to allow new smaller branches to take its place.  Often the case may be that a bonsai has a branch that was preferred many years ago but tastes or ideas change, this will be removed (sacrificed) to allow a new branch to take its place to offer a new shape.   The simplest way to keep a bonsai small is to simply cut all new growth back to a maximum size, this bonsai while remaining a small tree will have no shape and will eventually take on a ball like appearance, to get the best value from your bonsai and to be truly proud of it in 10 to 20 years it is best to look at the branches and remove those that appear out of place and encourage those that look good.  Branches can be shaped by cutting or by bending and holding in place for a few months with a safe wire.

When does a Bonsai need to be repotted
Bonsai can be created by cutting back and training regularly, they can also be created by neglect and starvation, however a starved plant will not live for ever and will not look very attractive.  To grow a proper bonsai and to create that amazing miniature tree effect takes many years of growing, the faster the bonsai is grown the quicker the tree will take shape.  Because of this regular repotting, feeding and watering are essential.  The idea is to grow the plant well and train it regularly.  If the plant is in a bonsai pot then most likely the decision has been made to leave this plant in this pot for many years, repotting will therefore mean that the plant must be removed from the pot, roots teased open and most of the finer roots removed.  This is an essential tool in the art of bonsai not to help keep the plant small but to allow it to grow more healthy vigorous roots and to allow it again to grow in the same size pot.  For the next 12 months after repotting the plant will grow well and will again slow down when it runs out of food or when the pot is to full of roots and new roots cannot form.  The only time a bonsai is potted into a bigger pot is if is size is out of proportion to the existing pot or the new design asks for a larger pot.  Many bonsai are grown in trainer pots or even in the ground for many years until the desired basic shape has been reached and only then does it go into a bonsai pot.  Plants can be removed from bonsai pots and placed into the ground for a year or two to thicken the stems. 

What are the best plants to bonsai
The best plants are those that are easy to look after, happy to be trimmed and have roots aggressively cut and have small leaves.  A plant with a 15cm leaf would look silly in a 15cm pot.  Most conifers and pines make great bonsai due to their needle like leaves.  Elms and maples make great bonsai as almost anything can be done to them.   Ficus make great indoor bonsai and will also do very well out doors, figs are probably the best beginners bonsai as they are almost fail safe and can survive short dry out periods.

Plants suitable for Bonsai
Acer buergerianum   Trident Maple   B, E, P, C, D
Acer palmatum  Japanese Maple  B, E, P, C, D
Acer palmatum v atropurpureum  Red Japanese Maple  B, E, P, C, D
Acer palmatum v atropurpureum disectum  Cut Leaf Maple  B, E, P, C, D
Almus parvifolia 'Suber'  Corky Bark Elm  B, E, P, D
Banksia integrifolia  Coastal Banksia  E
Banksia serrata  Old Man Banksia  E
Betula pendula  Silver Birch  B, P, C, D
Buxus microphylla  English Box  B, P
Buxus sempervirens  Dutch Box  B, P
Cedrus atlantica  Atlantic Cedar  P, E, C
Cedrus deodora  Deodara Cedar  P, E, C
Chamaecyparis obtusa kosteri     B, P, C
Chamaecyparis pisifera  Boulivard  B, P, C
Chameacyparis obtusa nana  The False Cyparis  B, P, E, C
Coccinnia royallii  Pyracantha  B, P, D, F, B
Cotonester microphyllus  Cotoneaster  B, P, E, F, B
Cratagus monogyna  Hawthorn  B, P, F, D, B
Crypromeria japonica 'Bandi Sugi'  Japanese Red Cedar  P, E, C,
Fagus silvatica  Beech  P, E, C, D
Fagus silvatica  Copper Beech  P, E, C, D
Ficus benjamina  Dwarf Fig  B, P, E, I, S, xx
Ficus macrophylla  Morton Bay Fig  B, P, E, I, S, x, A
Ficus microcarpa retusa  Banyan Fig  B, P, E, I, S, x
Ficus rubiginosa  Port Jackson Fig  B, P, E, I, S, x
Fraxinus americana  White Ash  B, P, E, D, C
Hargenbergia violacea 'Mini Halta'  Coral Pea  
Hebe 'Lake'  Hebe 'Lake'  
Juniper chinense  Chinese Juniper  B, P, E, C
Juniperus procumbens nana  Juniper  B, P, E, C
Juniperus sargentii 'Shimpaku'  Sargent Juniper  B, P, E, C
Juniperus squamata prostrart 'Pingii'  Juniper  B, P, E, C
Leptospermum laevigatum  Coastal Tea Tree  B, P, E, F
Ligustrum tuscany  Privit  
Ligustrum vulgare  Privit  
Liquidamber styracoflua  Sweet Gum  B, P, E, D, C
Lonicera Nirida 'Dwarf'  Honey Suckle  
Olea europaea  Olive  B, P, E
Parleyensis White  Erica  B, P, E, A
Picea abies (Little Gem)  Picea Conical  B, P, E, C
Picea albertiana  Dwarf Picea  B, P, E, C
Pieris japonica  Lilly of the Valley  B, P, E, S
Pieris japonica  Pieris Spring Candy  B, P, E, S
Pinus densiflora  Scots Pine  P, E, C
Pinus mugo  Dwarf Mountain Pine  B, P, E, C
Pinus parviflora  Japanese White Pine  B, P, E, C
Pinus sylvestris  Red Pine  P, E, C
Pinus thumbergii  Japanese Black Pine  P, E, C
Pittosporum 'Green Globe'  Green Globe  B, P
Pittosporum tenuifolium   B, P
Pyrus ussuriensis  Manchurian Pear  B, P, E, D, Berr
Quercus cerris  Turkish Oak  B, P, E, D, C
Quercus robur  English Oak  B, P, E, D, C
Rhododendron indica  Azalea Sakuki  B, P, E, F
Rhododendron Sao-to-me  Mini Azalea  B, P, E, F
Salix gracilistyla  Pussy Willow  B, P, E, D, C
Sequoia sempervirens  Californian Redwood  B, P, E, D, C
Serissa foetida  Kyoto 'Purple'  B, P, E, X, F
Serissa foetida  Snow Flowers  B, P, E, X, F
Syzyciium paniculatum  Lilly Pilly  ???
Taxodium disrichum  Swamp Cyparis  B, P, E, D, C
Thyhus serphyllum  Lemon Variegated Thyme  B, P, E, X
Ugni molinae  Chilli Guava  B, P, E, X
Ulmus elegantissima  Jacqueline Hillier  B, P, E, D, C
Ulmus parvifolia  Chinese Elm  B, P, E, D, C
Wisteria floribunda  Japanese Wisteria  B, P, E, D, C
Wisteria sinensis  Chinese Wisteria  B, P, E, D, C
Zelkova serrata  Japanese Elm  B, P, E, D, C

Note:
B -  Beginners. These are hardy easily cared for bonsai. 
P - Popular, regularly used by bonsai growers.
E - Expert, popular for collectors.
I -  Indoors, plant will grow well indoors.
S - Shade, plant requires some shade in summer.
X - Frost sensitive, protect in severe frosts.
XX - Frost free zones or warmer climates.
C - Cold climate, may need winter temperatures for dormancy
D - Deciduous, will loose leaves in winter.
F - Grown for flowers.